A sting in the mosquito tale

For over a decade, entomologists — people who study insects — have been noticing a peculiar trend, a drastically decreasing mosquito density per room but no major respite from the number of cases of vector borne diseases — principally dengue, transmitted by the Aedes aegypti mosquito, and malaria by the female Anopheles mosquito.Himansshu Bhatt&Ishita Bhadra | TNN | October 19, 2015, 06:03 IST

Climate change could accelerate the emergence of mosquito-borne diseases such as chikungunya, dengue fever, and West Nile virus in the UK within the next few decades.AHMEDABAD/SURAT/VADODARA: For over a decade, entomologists — people who study insects — have been noticing a peculiar trend, a drastically decreasing mosquito density per room but no major respite from the number of cases of vector borne diseases — principally dengue, transmitted by the Aedes aegypti mosquito, and malaria by the female Anopheles mosquito.

In Ahmedabad for instance, the average mosquito density (which includes the Aedes aegypti, Anopheles, Culex) in 1997 was almost 20 mosquitoes per room, while in 2015 it dropped to just 4 per room. This surprising trend has been witnessed in Surat as well. But state health officials claim that in case of dengue, relatively large densities of Aedes aegypti are not necessary for a large incidence of dengue cases. "Even temporal changes in population immunity or the introduction of new dengue serotype can significantly influence epidemic patterns and cause spikes," says a senior health official in Surat. Cities like Rajkot and Vadodara which do not have entomologists have also reported high incidence of dengue serotype 1, while in Ahmedabad all four serotypes of dengue was reported to be present.

A state health official observed: "Urban areas act as dengue epidemic 'pacemakers' because of their constant supply of new sets of population who are susceptible to dengue. Aedes aegypti mosquito here survives and efficiently transmits dengue virus even when Aedes aegypti densities are remarkably low." Experts suggest that even in low density, Aedes aegypti can transmit the dengue virus because of its propensity to imbibe frequent blood meals exclusively from humans many times during day time. This increases their contact with human hosts and as a result enhances their opportunities for contracting or transmitting the dengue virus infection.

In Vadodara, the Vadodara Municipal Corporation (VMC) health officer Dr Devesh Patel observes, "The monsoon was scanty in the city and there were very few breeding grounds for mosquitoes this year. But it is true that the state has witnessed a more virulent attack this year." (with inputs from Paul John)

Vadodara saw 44% increase in dengue cases:

With dengue cases still trickling in, Vadodara this year recorded a rise of over 44% in dengue cases. September alone saw as many as 120 cases, taking the total number since July 1 to 214. Authorities claim that the rise in numbers is the result of a fine-comb surveillance pattern and increased number of trained field health workers. Dr Devesh Patel, health officer for VMC told TOI that the city had no data available about the density of mosquitoes in the city or its relation to the rise and fall of vector-borne diseases.

"The city lacks the machinery to determine the most prevalent strain of dengue. Also, since it does not affect the clinical treatment of the patients, it is not on the authority's priority list," said head of the department of microbiology at SSG Hospital Dr Tanuja Javdekar. However, Javdekar claimed that random sample testing showed that DENV1 serotype affects most patients in the city.

Different strain of virus seen in Surat:

The city has reported a high prevalence of dengue serotype DENV2 virus according to the recent reports of the National Institute of Virology (NIV) in Pune.

"At present, what we see in the city is DENV2 virus. Once someone is infected by this virus and treated, he gets lifelong immunity," said Dr Keshav Vaishnav, in-charge of Vector-Borne Disease Control (VBDC) department of Surat Municipal Corporation (SMC).

At present, Aedes aegypti density in the city is 0.95 per man hour (MHD) which is lower than last year but has resulted in higher number of positive cases when compared to last year.

In 2013, the density of mosquitoes in the city was 0.98 MHD, in 2014 it was 1.02 MHD and till October 13, 2015 it is registered as 0.95 MHD

However in 2010, different strains like DenV3 and DenV4 were found in some areas (West zone) from Aedes aegypti mosquitoes. These were not found in humans.

Only in India the healthcare financing is very small when compared to the financing by the other forces rather than the patient himself or herself paying out of pocket. Having 70-75% of the expenses as out-of-pocket, in my opinion, is not a right approach to managing healthcare in a country where the patients tend to sub-optimally purchase healthcare if he/she has to pay out-of-pocket.